Antibacterial mechanisms of graphene-based composite nanomaterials

Extraction of Graphene Nanostructures from Colocasia esculenta and Nelumbo nucifera Leaves and Surface Functionalization with Tin Oxide: Evaluation of Their Antibacterial Properties

Expeditious evolution of antimicrobial resistance in recent years has been identified as a growing concern by various health organizations around the world. Herein, facile and environmentally benign production of highly antibacterial carbonaceous nanomaterials from Colocasia esculenta and Nelumbo nucifera leaves is reported. After carbonization and oxidative treatment, smaller graphene domains are formed in Colocasia esculenta derivatives, whereas larger sheetlike structures are observed in the case of Nelumbo nucifera. Smaller particle size makes quantum confinement effects more prominent, as is evident in fine-tuning of the photoluminescence emission after each stage of treatment. The influence of precursor materials on the antibacterial properties of the nanosystems is also demonstrated. When microbiocidal activity was tested against model bacteria Pseudomonas aeruginosa, the nanocomposite derived from Colocasia esculenta leaves showed higher activity than the antibiotic drug clarithromycin (control) with a measured zone of inhibition of 40±0.5 mm. This is one of the highest values reported in comparison with plant-based carbon-silver nanosystems. Quantitative analysis revealed that the nanocomposite obtained from Colocasia esculenta leaves has antimicrobial efficacy equivalent to those of commercial antibiotic drugs and is able to eradicate bacteria at much lower concentrations than that obtained from Nelumbo nucifera leaves.

Antimicrobial Mechanisms and Effectiveness of Graphene and Graphene-Functionalized Biomaterials. A Scope Review

Bacterial infections represent nowadays the major reason of biomaterials implant failure, however, most of the available implantable materials do not hold antimicrobial properties, thus requiring antibiotic therapy once the infection occurs. The fast raising of antibiotic-resistant pathogens is making this approach as not more effective, leading to the only solution of device removal and causing devastating consequences for patients. Accordingly, there is a large research about alternative strategies based on the employment of materials holding intrinsic antibacterial properties in order to prevent infections. Between these new strategies, new technologies involving the use of carbon-based materials such as carbon nanotubes, fullerene, graphene and diamond-like carbon shown very promising results. In particular, graphene- and graphene-derived materials (GMs) demonstrated a broad range antibacterial activity toward bacteria, fungi and viruses. These antibacterial activities are attributed mainly to the direct physicochemical interaction between GMs and bacteria that cause a deadly deterioration of cellular components, principally proteins, lipids, and nucleic acids. In fact, GMs hold a high affinity to the membrane proteoglycans where they accumulate leading to membrane damages; similarly, after internalization they can interact with bacteria RNA/DNA hydrogen groups interrupting the replicative stage. Moreover, GMs can indirectly determine bacterial death by activating the inflammatory cascade due to active species generation after entering in the physiological environment. On the opposite, despite these bacteria-targeted activities, GMs have been successfully employed as pro-regenerative materials to favor tissue healing for different tissue engineering purposes. Taken into account these GMs biological properties, this review aims at explaining the antibacterial mechanisms underlying graphene as a promising material applicable in biomedical devices.

Engineering of 2D Ti3C2 MXene Surface Charge and Its Influence on Biological Properties

Current trends in the field of MXenes emphasize the importance of controlling their surface features for successful application in biotechnological areas. The ability to stabilize the surface properties of MXenes has been demonstrated here through surface charge engineering. It was thus determined how changing the surface charges of two-dimensional (2D) Ti₃C₂ MXene phase flakes using cationic polymeric poly-L-lysine (PLL) molecules affects the colloidal and biological properties of the resulting hybrid 2D nanomaterial. Electrostatic adsorption of PLL on the surface of delaminated 2D Ti₃C₂ flakes occurs efficiently, leads to changing an MXene's negative surface charge toward a positive value, which can also be effectively managed through pH changes. Analysis of bioactive properties revealed additional antibacterial functionality of the developed 2D Ti₃C₂/PLL MXene flakes concerning Escherichia. coli Gram-negative bacteria cells. A reduction of two orders of magnitude of viable cells was achieved at a concentration of 200 mg L^-1. The in vitro analysis also showed lowered toxicity in the concentration range up to 375 mg L^-1. The presented study demonstrates a feasible approach to control surface properties of 2D Ti₃C₂ MXene flakes through surface charge engineering which was also verified in vitro for usage in biotechnology or nanomedicine applications.

Effective killing of bacteria under blue-light irradiation promoted by green synthesized silver nanoparticles loaded on reduced graphene oxide sheets

Graphene oxide (GO) materials loaded with silver nanoparticles (AgNPs) have drawn considerable attention due to their capacity to efficiently inactivate bacteria though a multifaceted mechanism of action, as well as for presenting a synergetic effect against bacteria when compared to the activity of AgNPs and GO alone. In this investigation, we present an inexpensive and environmentally-friendly method for synthesizing reduced GO sheets coated with silver nanoparticles (AgNPs/r-GO) using a coffee extract solution as a green reducing agent. The physical and chemical properties of the produced materials were extensively characterized by scanning electron microscopy (SEM), field-emission gun transmission electron microscopy (FEG-TEM), ultraviolet and visible absorption (UV-Vis), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), inductively coupled plasma-optical emission spectroscopy (ICP-OES) and ion release determination. The results demonstrated that AgNPs/r-GO composites were successfully produced, revealing the formation of micrometer-sized r-GO sheets decorated by AgNPs of approximately 70 nm diameter. Finally, bactericidal and photobactericidal effects of the AgNPs/r-GO composites were tested against Staphylococcus aureus, in which the results showed that the composites presented antimicrobial and photoantimicrobial activities. Moreover, our results demonstrated for the first time, to our knowledge, that an efficient process of bacterial inactivation can be achieved by using AgNPs/r-GO composites under blue light irradiation as a result of three different bacterial killing processes: (i) chemical effect promoted by Ag⁺ ion release from AgNPs; (ii) photocatalytic activity induced by AgNPs/r-GO composites, enhancing the bacterial photoinactivation due to the excited-Plasmons of the AgNPs when anchored on r-GO; and (iii) photodynamic effect produced by bacterial endogenous photosensitizers under blue-light irradiation. In summary, the present findings demonstrated that AgNPs/r-GO can be obtained by a non-toxic procedure with great potential for biomedical-related applications.

Antimicrobial activity of graphene oxide quantum dots: impacts of chemical reduction

Design and engineering of graphene-based functional nanomaterials for effective antimicrobial applications has been attracting extensive interest. In the present study, graphene oxide quantum dots (GOQDs) were prepared by chemical exfoliation of carbon fibers and exhibited apparent antimicrobial activity. Transmission electron microscopic measurements showed that the lateral length ranged from a few tens to a few hundred nanometers. Upon reduction by sodium borohydride, whereas the UV-vis absorption profile remained largely unchanged, steady-state photoluminescence measurements exhibited a marked blue-shift and increase in intensity of the emission, due to (partial) removal of phenanthroline-like structural defects within the carbon skeletons. Consistent results were obtained in Raman and time-resolved photoluminescence measurements. Interestingly, the samples exhibited apparent, but clearly different, antimicrobial activity against Staphylococcus epidermidis cells. In the dark and under photoirradiation (400 nm), the as-produced GOQDs exhibited markedly higher cytotoxicity than the chemically reduced counterparts, likely because of (i) effective removal by NaBH₄ reduction of redox-active phenanthroline-like moieties that interacted with the electron-transport chain of the bacterial cells, and (ii) diminished production of hydroxyl radicals that were potent bactericidal agents after chemical reduction as a result of increased conjugation within the carbon skeletons.

Exposure of Smaller and Oxidized Graphene on Polyurethane Surface Improves its Antimicrobial Performance

Catheter-related infections are a common worldwide health problem, highlighting the need for antimicrobial catheters. Here, antibacterial potential of graphene nanoplatelets (GNP) incorporated in the commonly used polymer for catheter manufacture-polyurethane (PU)-is investigated. Two strategies are explored: melt-blending, producing a composite, and dip coating, where a composite layer is deposited on top of PU. GNP with different lateral sizes and oxidation degrees-GNP-M5, GNP-M15, GNP-M5ox, GNP-M15ox-are applied in both strategies, and the antimicrobial potential towards Staphylococcus epidermidis of GNP dispersions and GNP-containing PU evaluated. As dispersions, oxidized and smaller GNP powders (GNP-M5ox) inhibit 74% bacteria growth at 128 µg/mL. As surfaces, GNP exposure strongly impacts their antimicrobial profile: GNP absence at the surface of composites yields no significant effects on bacteria, while by varying GNP: PU ratio and GNP concentration, coatings enhance GNP exposure, depicting an antimicrobial profile. Oxidized GNP-containing coatings induce higher antibacterial effect than non-oxidized forms, particularly with smaller GNPox, where a homogeneous layer of fused platelets is formed on PU, leading to 70% reduction in bacterial adhesion and 70% bacterial death. This pioneering work unravels how to turn a polymer clinically used to produce catheters into an antimicrobial surface, crucial to reducing risk of infection associated with catheterization.

Novel Poly(vinyl alcohol)/Chitosan/Modified Graphene Oxide Biocomposite for Wound Dressing Application

Rapid absorption of wound exudate and prevention of wound infection are prerequisites for wound dressing to accelerate wound healing. In this study, a novel kind of promising wound dressing is developed by incorporating polyhexamethylene guanidine (PHMG)-modified graphene oxide (mGO) into the poly(vinyl alcohol)/chitosan (PVA/CS) matrix, conferring the dressing the required mechanical properties, higher water vapor transmission rate (WVTR), less swelling time, improved antibacterial activity, and more cell proliferation compared to the PVA/CS film crosslinked by genipin. In vivo experiments indicate that the PVA/CS/mGO composite film can accelerate wound healing via enhancement of the re-epithelialization. PVA/CS/mGO composite film with 0.5 wt% mGO sheets displays the best wound healing properties, as manifested by the 50% higher antibacterial rate compared to GO and the wound healing rate of the mouse using this dressing is about 41% faster than the control group and 31% faster than the pure PVA/CS dressing. The underlying mechanism of the accelerated wound healing properties may be a result of the improved antibacterial ability to eradicate pathogenic bacteria on the wound area and maintain an appropriate moist aseptic wound healing environment to accelerate re-epithelialization. These findings suggest that this novel composite PVA/CS/mGO film may have promising applications in wound dressing.

Chondroinductive Alginate-Based Hydrogels Having Graphene Oxide for 3D Printed Scaffold Fabrication

Scaffolds based on bioconjugated hydrogels are attractive for tissue engineering because they can partly mimic human tissue characteristics. For example, they can further increase their bioactivity with cells. However, most of the hydrogels present problems related to their processability, consequently limiting their use in 3D printing to produce tailor-made scaffolds. The goal of this work is to develop bioconjugated hydrogel nanocomposite inks for 3D printed scaffold fabrication through a micro-extrusion process having improved both biocompatibility and processability. The hydrogel is based on a photocrosslinkable alginate bioconjugated with both gelatin and chondroitin sulfate in order to mimic the cartilage extracellular matrix, while the nanofiller is based on graphene oxide to enhance the printability and cell proliferation. Our results show that the incorporation of graphene oxide into the hydrogel inks considerably improved the shape fidelity and resolution of 3D printed scaffolds because of a faster viscosity recovery post extrusion of the ink. Moreover, the nanocomposite inks produce anisotropic threads after the 3D printing process because of the templating of the graphene oxide liquid crystal. The in vitro proliferation assay of human adipose tissue-derived mesenchymal stem cells (hADMSCs) shows that bioconjugated scaffolds present higher cell proliferation than pure alginate, with the nanocomposites presenting the highest values at long times. Live/Dead assay otherwise displays full viability of hADMSCs adhered on the different scaffolds at day 7. Notably, the scaffolds produced with nanocomposite hydrogel inks were able to guide the cell proliferation following the direction of the 3D printed threads. In addition, the bioconjugated alginate hydrogel matrix induced chondrogenic differentiation without exogenous pro-chondrogenesis factors as concluded from immunostaining after 28 days of culture. This high cytocompatibility and chondroinductive effect toward hADMSCs, together with the improved printability and anisotropic structures, makes these nanocomposite hydrogel inks a promising candidate for cartilage tissue engineering based on 3D printing.

Structure-Property-Toxicity Relationships of Graphene Oxide: Role of Surface Chemistry on the Mechanisms of Interaction with Bacteria

Graphene oxide (GO) is an antimicrobial agent with tunable surface chemistry. To identify the physicochemical determinants of GO's antimicrobial activity, we generated different modified Hummer's GO materials thermally annealed at 200, 500, or 800 °C (TGO200, TGO500, and TGO800, respectively) to modify the surface oxygen groups on the material. Plating assays show that as-received GO (ARGO) and TGO200, TGO500, and TGO800 reduce Escherichia coli viability by 50% (EC₅₀) at 183, 143, 127, and 86 μg/mL, respectively, indicating higher bacterial toxicity as ARGO is reduced. To uncover the toxicity mechanism of GO, fluorescent dye-based assays were used to measure oxidative stress at the EC₅₀. ARGO showed an increase in intracellular reactive oxygen species, measured as an increase in 2',7'-dichlorodihydrofluorescein diacetate fluorescence, whereas TGO500 and TGO800 induced an increase in the fluorescence of fluorescein diacetate (FDA) by 30 and 42%, suggesting a decrease in cell permeability. Because of a possible wrapping mechanism, plating assays after post-exposure sonication were performed to explain TGO's low oxidative response and high FDA levels. Results show no difference in colony-forming units, indicating that inhibition of cell growth is a result of the adsorption of bacterial cells on the GO material. By comparing different GO samples at their EC₅₀, this study reveals that reduction of GO alters both the mechanisms of cellular interaction and the degree of toxicity to bacteria.

Nanomaterials as a new opportunity for protecting workers from biological risk

Healthcare-Associated Infections (HAIs) represent a frequent complication for hospitalized patients and more rarely for workers. In recent years, substantial scientific evidence has been reached regarding the role played by the inanimate surfaces, especially those touched in patient-care areas, in the transmission of nosocomial pathogens. Therefore, it is essential to find new collective protective measures to minimize microbial contamination in healthcare facilities, thereby preventing the spread of multi-drug resistant bacteria. We present an overview of the major nano-enabled AntiMicrobial Coatings (AMCs) which may be used as collective protective measures in healthcare setting, discussing also some aspects related to their effectiveness and safety. AMCs may be classified within three groups on base of their mechanism of action: surfaces releasing active compound, contact-killing surfaces and anti-adhesive surfaces. To date, little information is available on the effectiveness of AMCs to reduce the risk of HAIs since the most of studies do not reach conclusive results on their beneficial effects. Moreover, the lack of standard protocols for assessing antimicrobial efficacy and poor data about the interaction between AMCs and disinfectants prevent their placing on the market. Further studies are needed for assessing risks and benefits of AMCs as collective protective measures in healthcare setting.

Photothermal Killing of Methicillin-Resistant Staphylococcus aureus by Bacteria-Targeted Polydopamine Nanoparticles with Nano-Localized Hyperpyrexia

Bacterial infections caused by antibiotic-resistant pathogens have become intractable problems to public health. Therefore, there is an imperious demand for developing new approaches to effectively kill antibiotic-resistant bacteria. In this work, we report a kind of bacteria-targeted polydopamine nanoparticle exhibiting great photothermal killing ability toward methicillin-resistant Staphylococcus aureus (MRSA) by nano-localized hyperpyrexia under low-power near-infrared (NIR) light irradiation. These bacteria-targeted nanoparticles (PDA-PEG-Van) are prepared by modifying polydopamine nanoparticles with thiol-poly(ethylene glycol) (mPEG-SH) and vancomycin (Van) molecules. The PEG shell endows the nanoparticles with excellent long-term circulation stability. Due to the multivalent hydrogen-bond interactions between vancomycin and the MRSA cell wall, the vancomycin-modified polydopamine nanoparticles can specifically target MRSA rather than mammalian cells. These bacteria-targeted nanoparticles are employed as a nano-localized heat source to kill MRSA via disrupting the bacterial cell wall and membrane under irradiation of low-power NIR light. More importantly, the surrounding healthy tissues suffer bare damage, owing to the absence of any targeting effect of PDA-PEG-Van toward mammalian cells and the low power of NIR light used in the therapeutic process. Given the above advantages, the bacteria-targeted polydopamine nanoparticles proposed in this work show tremendous potential to treat MRSA infections, because they can effectively limit localized heating in the infection sites to kill bacteria and cut down damage to healthy tissues.

Graphene oxide nanohybrids for electron transfer-mediated antimicrobial activity

The rapid increase in the prevalence of antibiotic-resistant bacterial strains poses a global health risk. In this scenario, alternative strategies are needed to combat the alarming rise in multidrug-resistant bacterial populations. For example, metal-incorporated graphene derivatives have emerged as model nanomaterials owing to their intrinsic antibacterial activity together with their biocompatibility. Interestingly, photon-activated phthalocyanine sensitizers have also shown promising physiochemical biocidal effects against pathogenic bacteria populations when conjugated with diverse nanomaterials. Herein, we report the facile synthesis of graphene oxide incorporated zinc phthalocyanine (ZnPc-GO) nanohybrids showing bactericidal activity against Gram-negative Escherichia coli (E. coli) cells, in the absence of any photo-excitation. The ZnPc-GO hybrid nanomaterials were synthesized by the in situ deposition of GO flakes on ZnPc-coated indium tin oxide (ITO) substrates. Two types of morphologically different ZnPc molecules, potato-chip-like α-phase ZnPc, namely ZnPc(A), and nanorod-like β-phase ZnPc(B), were used for the synthesis of the ZnPc(A/B)-GO nanocomposites. The interactions of GO with the underlying ZnPc(A/B) entities in the ZnPc-GO systems were investigated using multiple characterization techniques. It was observed that the GO flakes in the ZnPc(B)-GO nanocomposite possess stronger π-π interactions and thus show a more efficient electron transfer mechanism when compared with the ZnPc(A) counterpart. Furthermore, the E. coli bacterial cells with an electronegative surface demonstrated a profound adherence to the electron-withdrawing ZnPc(B)-GO surface. The death kinetics of bacteria with ZnPc(B)-GO were further investigated using surface potential mapping and Kelvin probe force microscopy (KPFM) analysis. Upon direct contact with ZnPc(B)-GO, the adhered bacterial cells showed outer cell deformation and membrane protein leakage, induced by a proposed charge-transfer mechanism between negatively charged cells and the electron-withdrawing ZnPc(B)-GO surface. These new findings may provide insights into the design of potential ZnPc-GO-based novel antimicrobial nanomaterials or surface coatings.

Superior antibacterial activity of ZnO-CuO nanocomposite synthesized by a chemical Co-precipitation approach

Synthesis of highly efficient antibacterial agents has become highly important due to emergence of antibiotic resistance. Herein, Pristine ZnO and ZnO-CuO nanocomposite has been synthesized by simple chemical co-precipitation method and characterized by X-ray diffraction (XRD), microscopic and spectroscopic techniques. The prepared ZnO-CuO nanocomposite is composed of two dimensional nanosheets consisting of hexagonal ZnO and monoclinic CuO crystal phases present in coexistence. Moreover, a minute presence of Cu₅Zn₈ cubic phase has been evident in the XRD pattern of ZnO-CuO nanocomposite. Fourier Transform Infrared Spectroscopy (FTIR) spectrum of the prepared nanocomposite has revealed the presence of vibrational modes related to both Zn-O and Cu-O. Photoluminescence (PL) investigations depicted the formation of huge amounts of surface defects in ZnO-CuO nanocomposite as compared to pristine ZnO nanostructures. The prepared ZnO-CuO nanocomposite has efficiently killed Methicillin resistant Staphylococus aureus (s. aureus) bacterium by producing 24 mm of zone of inhibition (ZOI) comparing to 8 mm ZOI produced by pristine ZnO. The superior antibacterial activity of ZnO-CuO nanocomposite has been attributed to oxidative stress generated by electron transfer pathway and reactive oxygen species (ROS) generation.

Nanocarbon materials in water disinfection: state-of-the-art and future directions

Water disinfection practices are critical for supplying safe drinking water. Existing water disinfection methods come with various drawbacks, calling for alternative or complementary solutions. Nanocarbon materials (NCMs) offer unique advantages for water disinfection owing to their high antimicrobial activity, often low environmental/human toxicity, and tunable physicochemical properties. Nevertheless, it is a challenge to assess the research progress made so far due to the structure and property diversity in NCMs as well as their different targeted applications. Because of these, here we provide a broad outline of this emerging field in three parts. First, we introduce the antimicrobial activities of the different types of NCMs, including fullerenes, nanodiamonds, carbon (nano)dots, carbon nanotubes, and graphene-family materials. Next, we discuss the current status in applying these NCMs for different water disinfection problems, especially as hydrogel filters, filtration membranes, recyclable aggregates, and electrochemical devices. We also introduce the use of NCMs in photocatalysts for photocatalytic water disinfection. Lastly, we put forward the key hurdles of the field that hamper the realization of the practical applications and propose possible directions for future investigations to address those. We hope that this minireview will encourage researchers to tackle these challenges and innovate NCM-based water disinfection platforms in the near future.

Thermoresponsive Amphiphilic Functionalization of Thermally Reduced Graphene Oxide to Study Graphene/Bacteria Hydrophobic Interactions

An understanding of the interactions of 2D nanomaterials with pathogens is of vital importance to developing and controlling their antimicrobial properties. In this work, the interaction of functionalized graphene with tunable hydrophobicity and bacteria is investigated. Poly(ethylene glycol)- block-(poly- N-isopropylacrylamide) copolymer (PEG- b-PNIPAM) with the triazine joint point was attached to the graphene surface by a nitrene [2 + 1] cycloaddition reaction. By thermally switching between hydrophobic and hydrophilic states, functionalized graphene sheets were able to bind to bacteria. Bacteria were eventually disrupted when the functionality was switched to the hydrophobic state. On the basis of measuring the different microscopy methods and a live/dead viability assay, it was found that Escherichia coli ( E. coli) bacteria are more susceptible to hydrophobic interactions than B. cereus bacteria, under the same conditions. Our investigations confirm that hydrophobic interaction is one of the main driving forces at the presented graphene/bacteria interfaces and promotes the antibacterial activity of graphene derivatives significantly.

Facile synthesis of graphene-tin oxide nanocomposite derived from agricultural waste for enhanced antibacterial activity against Pseudomonas aeruginosa

Antibacterial screening of graphene-tin oxide nanocomposites synthesized from carbonized wood and coconut shell is investigated against Pseudomonas aeruginosa for the first time. Efficient and facile one step hydrothermal process adopted in the present work for the synthesis of graphene-tin oxide nanoparticles provides an ideal method for the economic large-scale production of the same. Graphene-tin oxide nanocomposites derived from wood charcoal possess a spherical morphology whereas rod like structures are seen in the case of coconut shell derivatives. An excitation independent fluorescence response is observed in graphene-tin oxide nanohybrids while graphene oxide nanostructures exhibited an excitation dependent behavior. These hydrophilic nanostructures are highly stable and exhibited no sign of luminescence quenching or particle aggregation even after a storage of 30 months. Bactericidal effects of the nanostructures obtained from coconut shell is found to be relatively higher compared to those procured from wood. This variation in antibacterial performance of the samples is directly related to their morphological difference which in turn is heavily influenced by the precursor material used. MIC assay revealed that coconut shell derived graphene-tin oxide composite is able to inhibit the bacterial growth at a lower concentration (250 μg/mL) than the other nanostructures. Nanocomposites synthesized from agro-waste displayed significantly higher antimicrobial activity compared to the precursor and graphene oxide nanostructures thereby making them excellent candidates for various bactericidal applications such as disinfectants, sanitary agents etc.

M. Biowaste derived graphene quantum dots interlaced with SnO2 nanoparticles – a dynamic disinfection agent against Pseudomonas aeruginosa

A biocidal GQD/SnO₂ nanocomposite derived from discarded sugarcane bagasse is a cost effective, renewable and green replacement for the traditional hazardous microbicides.

Graphene oxide and carbon dots as broad-spectrum antimicrobial agents - a minireview

Due to the increasing global population, growing contamination of water and air, and wide spread of infectious diseases, antibiotics are extensively used as a major antibacterial drug. However, many microbes have developed resistance to antibiotics through mutation over time. As an alternative to antibiotics, antimicrobial nanomaterials have attracted great attention due to their advantageous properties and unique mechanisms of action toward microbes. They inhibit bacterial growth and destroy cells through complex mechanisms, making it difficult for bacteria to develop drug resistance, though some health concerns related to biocompatibility remain for practical applications. Among various antibacterial nanomaterials, carbon-based materials, especially graphene oxide (GO) and carbon dots (C-Dots), are promising candidates due to the ease of production and functionalization, high dispersibility in aqueous media, and promising biocompatibility. The antibacterial properties of these nanomaterials can be easily adjusted by surface modification. They are promising materials for future applications against multidrug-resistant bacteria based on their strong capacity in disruption of microbial membranes. Though many studies have reported excellent antibacterial activity of carbon nanomaterials, their impact on the environment and living organisms is of concern due to the accumulatory and cytotoxic effects. In this review, we discuss antimicrobial applications of the functional carbon nanomaterials (GO and C-Dots), their antibacterial mechanisms, factors affecting antibacterial activity, and concerns regarding cytotoxicity.

Graphene oxide-silver nanocomposites modulate biofilm formation and extracellular polymeric substance (EPS) production

Biofilms with positive and negative actions ubiquitously affect medical infections, environmental remediation and industrial processes. However, it remains challenging to control the growth of harmful biofilms as well as to exploit the use of beneficial biofilms. Here we investigated the effect of an antibacterial graphene oxide-silver nanoparticles (GO-AgNPs) composite on Pseudomonas aeruginosa biofilm formation. We found that GO-AgNPs prevented biofilm formation in a dose-dependent manner, with a threshold of 15 μg mL^-1. Interestingly, the bacterial biomass significantly decreased, but extracellular polymeric substance (EPS) production remarkably increased in mature biofilms treated with GO-AgNPs of an appropriate concentration, suggesting that GO-AgNPs effectively modulate biofilm development and structure. Moreover, we established that GO-AgNPs caused bacterial death via both physical damage and oxidative stress, showing the synergic action of GO and AgNPs. These findings facilitate the use of graphene-based nanocomposites for greener antibiotic applications.

Powerful antibacterial activity of graphene/nanoflower-like nickelous hydroxide nanocomposites

Aim: The development of new and efficient antibacterial agents is urgent to overcome emerging antimicrobial resistance. Materials & methods: Herein, we have presented a new type of 3D antibacterial system to prompt bacteria to contact with the any plane of nanocomposites. Results: Comparing the antibacterial activity of graphene oxide, reduced graphene oxide and graphene-loaded nanoflower-like nickelous hydroxide (GN/Ni(OH)2) nanocomposites; the GN/Ni(OH)2 showed stronger bactericidal capability toward Gram-negative/-positive bacteria. Moreover, the GN/Ni(OH)2 with low cytotoxicity can promote it as ‘green’ antimicrobial agents. And, the GN/Ni(OH)2 presented long-term stable antibacterial effectiveness after 2-month storage. The antibacterial mechanisms of GN/Ni(OH)2 were evidenced as the 3D contact and violent damage to the bacterial structure. Conclusion: The GN/Ni(OH)2 provides new insights into the antibacterial properties of 3D nanocomposites for effectively fighting pathogen threats in biomedicine and public health.

All-in-one NIR-activated nanoplatforms for enhanced bacterial biofilm eradication

The chronic infection of humans by antibiotic-resistant bacteria and their related biofilm have, so far, not been properly addressed. In the present work, we developed a novel antibacterial nanoplatform showing the most efficient antibiotic-resistant bacteria inhibition and biofilm eradication. This particular formulation contains tobramycin-conjugated graphene oxide, for efficiently capturing bacteria through electrostatic interactions and eliminating bacteria as a "nano-knife", and copper sulphide nanoparticles for enhancing the photothermal and photodynamic properties. This novel formulation can selectively eliminate bacteria over NIH 3T3 cells, and the biofilm eradication capacity was up to 70%. Importantly, the nanoplatforms can inhibit bacterial growth and promote the repair of antibiotic-resistant bacteria-infected wounds on rats without non-specific damage to normal tissue. This work provides an effective, simple, and rapid method for the design and fabrication of near-infrared light-induced nanoplatforms that offer possibilities to treat biofilm-related infections.

Facile reduction of graphene oxide suspensions and films using glass wafers

This paper reports a facile and green method for conversion of graphene oxide (GO) into graphene by low-temperature heating (80 °C) in the presence of a glass wafer. Compared to conventional GO chemical reduction methods, the presented approach is easy-scalable, operationally simple, and based on the use of a non-toxic recyclable deoxygenation agent. The efficiency of the proposed method is further expanded by the fact that it can be applied for reducing both GO suspensions and large-scale thin films formed on various substrates prior to the reduction process. The quality of the obtained reduced graphene oxide (rGO) strongly depends on the type of the used glass wafer, and, particularly, magnesium silicate glass can provide rGO with the C/O ratio of 7.4 and conductivity of up to 33000 S*cm^-1. Based on the data obtained, we have suggested a mechanism of the observed reduction process in terms of the hydrolysis of the glass wafer with subsequent interaction of the leached alkali and alkali earth cations and silicate anions with graphene oxide, resulting in elimination of the oxygen-containing groups from the latter one. The proposed approach can be efficiently used for low-cost bulk-quantity production of graphene and graphene-based materials for a wide field of applications.

Antimicrobial graphene materials: the interplay of complex materials characteristics and competing mechanisms

Graphene materials (GMs) exhibit attractive antimicrobial activities promising for biomedical and environmental applications. However, we still lack full control over their behaviour and performance mainly due to the complications arising from the coexistence and interplay of multiple factors. Therefore, in this minireview, we attempt to illustrate the structure-property-activity relationships of GMs' antimicrobial activity. We first examine the chemical/physical complexity of GMs focusing on five aspects of their materials characteristics: (i) chemical composition, (ii) impurities and imperfections, (iii) lateral dimension, (iv) self-association (e.g., restacking), and (v) composite/hybrid formation. Next, we briefly summarise the current understanding of their antimicrobial mechanisms. Then, we assign the outlined materials characteristics of GMs to the proposed antimicrobial mechanisms. Lastly, we share our vision regarding the future of research and development in this fast-emerging field.

Graphene Materials in Antimicrobial Nanomedicine: Current Status and Future Perspectives

Graphene materials (GMs), such as graphene, graphene oxide (GO), reduced GO (rGO), and graphene quantum dots (GQDs), are rapidly emerging as a new class of broad-spectrum antimicrobial agents. This report describes their state-of-the-art and potential future covering both fundamental aspects and biomedical applications. First, the current understanding of the antimicrobial mechanisms of GMs is illustrated, and the complex picture of underlying structure-property-activity relationships is sketched. Next, the different modes of utilization of antimicrobial GMs are explained, which include their use as colloidal dispersions, surface coatings, and photothermal/photodynamic therapy agents. Due to their practical relevance, the examples where GMs function as synergistic agents or release platforms for metal ions and/or antibiotic drugs are also discussed. Later, the applicability of GMs in the design of wound dressings, infection-protective coatings, and antibiotic-like formulations ("nanoantibiotics") is assessed. Notably, to support our assessments, the existing clinical applications of conventional carbon materials are also evaluated. Finally, the key hurdles of the field are highlighted, and several possible directions for future investigations are proposed. We hope that the roadmap provided here will encourage researchers to tackle remaining challenges toward clinical translation of promising research findings and help realize the potential of GMs in antimicrobial nanomedicine.

Highly Stable Graphene-Based Nanocomposite (GO-PEI-Ag) with Broad-Spectrum, Long-Term Antimicrobial Activity and Antibiofilm Effects

Various silver nanoparticle (AgNP)-decorated graphene oxide (GO) nanocomposites (GO-Ag) have received increasing attention owing to their antimicrobial activity and biocompatibility; however, their aggregation in physiological solutions and the generally complex synthesis methods warrant improvement. This study aimed to synthesize a polyethyleneimine (PEI)-modified and AgNP-decorated GO nanocomposite (GO-PEI-Ag) through a facile approach through microwave irradiation without any extra reductants and surfactants; its antimicrobial activity was investigated on Gram-negative/-positive bacteria (including drug-resistant bacteria) and fungi. Compared with GO-Ag, GO-PEI-Ag acquired excellent stability in physiological solutions and electropositivity, showing substantially higher antimicrobial efficacy. Moreover, GO-PEI-Ag exhibited particularly excellent long-term effects, presenting no obvious decline in antimicrobial activity after 1 week storage in physiological saline and repeated use for three times and the lasting inhibition of bacterial growth in nutrient-rich culture medium. In contrast, GO-Ag exhibited a >60% decline in antimicrobial activity after storage. Importantly, GO-PEI-Ag effectively eliminated adhered bacteria, thereby preventing biofilm formation. The primary antimicrobial mechanisms of GO-PEI-Ag were evidenced as physical damage to the pathogen structure, causing cytoplasmic leakage. Hence, stable GO-PEI-Ag with robust, long-term antimicrobial activity holds promise in combating public-health threats posed by drug-resistant bacteria and biofilms.

Mechanisms of oxidative stress, apoptosis, and autophagy involved in graphene oxide nanomaterial anti-osteosarcoma effect

Background

Graphene and its derivative graphene oxide (GO) have been implicated in a wide range of anticancer effects.

Purpose

The objective of this study was to systematically evaluate the toxicity and underlying mechanisms of GO on two osteosarcoma (OSA) cancer cell lines, MG-63 and K₇M₂ cells.

Methods

MG-63 and K₇M₂ cells were treated by GO (0–50 µg/mL) for various time periods. Cell viability was tested by MTT and Live/Dead assays. A ROS Detection Kit based on DHE oxidative reaction was used for ROS detection. An Annexin V-FITC Apoptosis Kit was used for apoptosis detection. Dansylcadaverine (MDC) dyeing was applied for seeking unspecific autophagosomes. Western blot and Immunofluorescence analysis were used for related protein expression and location.

Results

K₇M₂ cells were more sensitive to GO compared with MG-63 cells. The mechanism was attributed to the different extent of the generation of reactive oxygen species (ROS). In K₇M₂ cells, ROS was easily stimulated and the apoptosis pathway was subsequently activated, accompanied by elevated expression of proapoptosis proteins (such as caspase-3) and decreased expression levels of antiapoptosis proteins (such as Bcl-2). A ROS inhibitor (N-acetylcysteine) could alleviate the cytotoxic effects of GO in K₇M₂ cells. However, the production of ROS in MG-63 cells was probably inhibited by the activation of an antioxidative factor, nuclear factor-E2-related factor-2, which translocated from the cytoplasm to the nucleus after GO treatment, while a nuclear factor-E2-related factor-2 inhibitor (ML385) significantly increased ROS production in MG-63 cells when combined with GO treatment. In addition, autophagy was simultaneously stimulated by characteristic autophagosome formation, autophagy flux, and increased the expression level of autophagy-related proteins (such as LC3I to LC3II conversion, ATG5, and ATG7).

Conclusion

This paper proposes various underlying mechanisms of the anticancer effect of GO. The novel synthetic use of GO with an oxidizing agent is the key step for further potential applications in clinical OSA cancer therapy.

pH-triggered charge-reversible of glycol chitosan conjugated carboxyl graphene for enhancing photothermal ablation of focal infection

Subcutaneous abscesses infected by multidrug-resistant bacteria are becoming an increasing challenge to human health. To address this challenge, a surface-adaptive and biocompatible glycol chitosan conjugated carboxyl graphene (GCS-CG) is developed, which exhibits unique self-adaptive target to the acidic microenvironment of abscess (∼pH 6.3) and no damage to the healthy tissue (pH 7.4) around the abscess. Originally, following conjugated with GCS, the absorbance of CG obviously increases in the near-infrared (NIR) region, enabling GCS-CG to generate an increment amount of heat. GCS-CG shows fast pH-responsive surface charge transition from negative to positive, which presents strong adherence to negatively charged bacteria surface in abscess, while exhibits poor affinity to host cells in healthy tissues. The local temperature of NIR-irradiated GCS-CG is estimated to be higher than their ambient temperature, ensuring targeted heating and eradicating the bacteria to reduce the damage to tissue; hence, wound healing is accelerated. Moreover, the in vitro and in vivo biosafety results demonstrate that GCS-CG presents greatly biocompatible even at a high concentration of 1 mg·mL^-1. Given the above advantages as well as the simple preparation, graphene developed here may provide a new potential application as a useful antibacterial agent in the areas of healthcare.

STATEMENT OF SIGNIFICANCE

A surface-adaptive nanomaterial, glycol chitosan conjugated carboxyl graphene (GCS-CG) is developed, which realizes the acidity-triggered bacteria targeting. GCS-CG can result in direct thermal ablation of bacteria and enhancement of the infected wound healing, but exhibit no damage to healthy tissues. The pH-responsive GCS-CG described here, containing no antibiotics, has great potentials in treating bacterial infection and even multidrug-resistant bacteria.

Mechanistic insight into the in vitro toxicity of graphene oxide against biofilm forming bacteria using laser-induced breakdown spectroscopy

While the cytotoxicity of graphene oxide (GO) has been well established, its bactericidal mechanism, however, has yet to be elucidated to advance GO-based biomedical and environmental applications. In an attempt to better understand the bactericidal action of GO, herein we studied the interactions of GO with Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus cells using physical techniques and chemical probes, respectively. In particular, a novel laser-induced breakdown spectroscopy (LIBS) based elemental fingerprint analysis revealed notable differences between viable and non-viable cells based on the difference in the concentration of trace inorganic elements in complex bacterial systems, which reflect cellular membrane integrity. Lower emission intensities from essential inorganic ions in the GO-treated cells offered explicit evidence on the efflux of intracellular molecules from the bacteria through damaged cell membranes. Furthermore, a detailed structural and morphological investigation of bacterial membrane integrity confirmed GO-induced membrane stress upon direct contact interactions with bacterial cells, resulting in the disruption of cellular membranes. Moreover, the generation of intracellular reactive oxygen species (ROS) in the presence of an added antioxidant underlined the role of GO-mediated oxidative stress in bacterial cell inactivation. Thus, by correlating the changes in the bacterial elemental compositions with the severe morphological alterations and the high ROS production witnessed herein, we propose that the bactericidal mechanism of GO is likely to be the synergy between membrane and oxidative stress towards both tested species. Our findings offer useful guidelines for the future development of GO-based antibacterial surfaces and coatings.

Surface modifications for antimicrobial effects in the healthcare setting: a critical overview

The spread of infections in healthcare environments is a persistent and growing problem in most countries, aggravated by the development of microbial resistance to antibiotics and disinfectants. In addition to indwelling medical devices (e.g. implants, catheters), such infections may also result from adhesion of microbes either to external solid-water interfaces such as shower caps, taps, drains, etc., or to external solid-gas interfaces such as door handles, clothes, curtains, computer keyboards, etc. The latter are the main focus of the present work, where an overview of antimicrobial coatings for such applications is presented. This review addresses well-established and novel methodologies, including chemical and physical functional modification of surfaces to reduce microbial contamination, as well as the potential risks associated with the implementation of such anticontamination measures. Different chemistry-based approaches are discussed, for instance anti-adhesive surfaces (e.g. superhydrophobic, zwitterions), contact-killing surfaces (e.g. polymer brushes, phages), and biocide-releasing surfaces (e.g. triggered release, quorum sensing-based systems). The review also assesses the impact of topographical modifications at distinct dimensions (micrometre and nanometre orders of magnitude) and the importance of applying safe-by-design criteria (e.g. toxicity, contribution for unwanted acquisition of antimicrobial resistance, long-term stability) when developing and implementing antimicrobial surfaces.

Antibacterial applications of graphene oxides: structure-activity relationships, molecular initiating events and biosafety

Bacterial infections may lead to diverse acute or chronic diseases (e.g., inflammation, sepsis and cancer). New antibiotics against bacteria are rarely discovered in recent years, which necessitates the exploration of new antibacterial agents. Engineered nanomaterials (ENMs) have been extensively studied for antibacterial use because of their long lasting killing effects in wide spectra of bacteria. Graphene oxide (GO) is one of the most widely studied ENMs and exhibit strong bactericidal effects. The physicochemical properties of GO play important roles in bacterial killing by triggering a cascade of toxic events. Many studies have explored the signaling pathways of GO in bacteria. Although molecular initiating events (MIEs) of GO in bacteria dominate its killing efficiency as well as toxicity mechanisms, they have been rarely reviewed. In this report, we discussed the structure-activity relationships (SARs) involved in GO-induced bacterial killing and the MIEs including redox reaction with biomolecules, mechanical destruction of membranes and catalysis of extracellular metabolites. Furthermore, we summarized the clinical or commercial applications of GO-based antibacterial products and discussed their biosafety in mammal. Finally, we reviewed the remaining challenges in GO for antibacterial applications, which may offer new insights for the development of nano antibacterial studies.

Synthesis of silver nanoparticle-decorated hydroxyapatite (HA@Ag) poriferous nanocomposites and the study of their antibacterial activities

Herein, we demonstrate a facile and green rapid approach for the synthesis of uniform poriferous hydroxylapatite [Ca₁₀(PO₄)₆(OH)₂, HA] and poriferous silver nanoparticle (Ag NPs)-decorated hydroxylapatite (HA@Ag) nanocomposites with excellent antibacterial properties. All the nanocomposites were fully characterized in the solid state via various techniques such as X-ray powder diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), automatic specific surface area and porosity analysis (BET) and field emission scanning electron microscopy (FESEM). The results show that HA has a porous rod-like structure, which the HA@Ag nanocomposites retained, and the surface of HA was loaded with globular-like Ag NPs with an average diameter of about 5.8 nm, which exhibit a well-crystalline state. The experimental parameters such as pH, the molar ratio of HA and Tollens' reagent, and reductant have a significant effect on the size and distribution of the Ag NPs. Moreover, the antimicrobial activities of HA and HA@Ag against Escherichia coli (E. coli), Pseudomonas aeruginosa (P. aeruginosa) and Staphylococcus aureus (S. aureus) were evaluated via broth dilution, filter paper diffusion, optical density (OD₆₀₀) and electron microscopy observation. The as-prepared HA@Ag nanocomposites exhibit excellent antibacterial activities, especially for S. aureus. The minimum inhibition concentration (MIC) of HA@Ag is only 3.9 μg mL⁻¹.

We demonstrate a facile and green rapid approach for the synthesis of uniform poriferous hydroxylapatite (HA) and poriferous silver nanoparticles (Ag NPs)-decorated hydroxylapatite (HA@Ag) nanocomposites with excellent antibacterial properties.

Photo-enhanced antibacterial activity of ZnO/graphene quantum dot nanocomposites

Synthesis of new, highly active antibacterial agents has become increasingly important in light of emerging antibiotic resistance. In the present study, ZnO/graphene quantum dot (GQD) nanocomposites were produced by a facile hydrothermal method and characterized by an array of microscopic and spectroscopic measurements, including transmission electron microscopy, X-ray photoelectron spectroscopy, UV-vis and photoluminescence spectroscopy. Antibacterial activity of the ZnO/GQD nanocomposites was evaluated with Escherichia coli within the context of minimum inhibitory concentration and the reduction of the number of bacterial colonies in a standard plate count method, in comparison to those with ZnO and GQD separately. It was found that the activity was markedly enhanced under UV photoirradiation as compared to that in ambient light. This was ascribed to the enhanced generation of reactive oxygen species under UV photoirradiation, with minor contributions from membrane damage, as manifested in electron paramagnetic resonance and fluorescence microscopic measurements. The results highlight the significance of functional nanocomposites based on semiconductor nanoparticles and graphene derivatives in the development of effective bactericidal agents.

NiO-nanoflakes grafted graphene: an excellent photocatalyst and a novel nanomaterial for achieving complete pathogen control

The increased levels of industrial pollutants in water and of drug-resistant pathogens more generally are a serious threat to human and aquatic life. Herein, we present the solar-light-induced dye removal and bactericidal properties of nickel oxide (NiO) and graphene nanoplatelet (GNP) nanocomposites. The conducting nature of GNPs is the key factor that accounts for the enhanced photocatalytic and antibacterial activity. Remarkably, the graphene/NiO nanocomposite shows outstanding photocatalytic activity (99% degradation) as compared to NiO (34%) alone, which makes it a potential candidate for the depollution of dye-contaminated water. In addition, the optimized amount of GNPs in the graphene/NiO nanocomposite renders it an exceptional antibacterial material, producing 100% growth inhibition of pathogenic microorganisms (both Gram-positive and Gram-negative bacteria). Therefore, the graphene/NiO nanocomposite can be an innovative material to achieve complete pathogen control, alongside being an economic solution for water treatment.

Aortic Graft Infection: Graphene Shows the Way to an Infection-Resistant Vascular Graft

Aortic graft infection is a potentially lethal complication of open and endovascular repair of aortic aneurysms. Graphene is the only existing two-dimensional material, and its unique structure gives graphene and its derivatives a plethora of original characteristics. Among other characteristics, graphene demonstrates bacteriostatic and bactericidal effects that could potentially resolve the problem of graft infection in the future. Data already exist in literature supporting this antibacterial effect of graphene oxide and reduced graphene oxide. Combining these materials with other substances enhances the antibacterial effect. Additionally, it looks feasible to expect antibiotic-delivering graphene-based graft materials in the future. Based on already published data, we could conclude that regarding graphene and its derivatives, the blessing of bactericidal effect comes with the curse of human cells toxicity. Therefore, it is important to find a fine balance between the desired antibacterial and the adverse cytotoxic effect before graphene is used in graft materials for humans.